A turbocharger comprises a drive shaft one end of which supports a turbine arranged to be driven by exhaust gases from an internal combustion engine. In automotive heavy duty diesel engines turbocharger shafts are supported in a housing usually by two separate floating bearings which are retained in position by circlips or some other conventional mechanical configuration. In a fully floating bearing, the shaft rotates relative to an inner bearing surface defined by a bearing body which also defines an outer bearing surface which itself rotates relative to a surrounding housing. The turbocharger shaft is generally located axially by a separate bearing. In a turbocharger the end of the shaft remote from the turbine simply drives a compressor which is used to deliver air to the engine.
In turbocompound engines, two turbines are provided in series, both driven by the exhaust gases of the engine. One of the turbines drives a compressor to deliver pressurised air to the engine and the other, a power turbine, is used to generate additional power which is transmitted via a mechanical connection. For example, in a power turbine a gear wheel may be fixed to the end of the shaft remote from the turbine and the gear wheel is used to transmit power into an appropriate coupling, for example a fluid coupling or other drive mechanism into the crankshaft of the engine. The power may however be transmitted by other means, for example hydraulically or electrically.
In a power turbine, in which additional power generated is fed back into the crankshaft of the engine via a gear wheel on the turbine shaft, different loadings are applied to the shaft bearing system as compared with loadings in a conventional turbocharger which does no more than drive a compressor. In a conventional turbocharger, out of balance forces and shaft vibration forces are resisted by journal oil films distributed equally around the circumference of the inner and outer bearings as there are no off axis external forces on the system. In a power turbine in contrast, the gear drive supported on the end of the shaft remote from the turbine generates a reaction force which gives rise to an external directional force on the turbine shaft. This external force significantly increases the load, particularly on the bearing closest to the gear. As a result conventional floating bearing arrangements are not suitable for use at the gear end (or other drive connection) of a power turbine. This is because load carrying oil films in fully floating bearings require relative rotation at both the inner and outer bearing surfaces. However, the directional load at the gear end of the shaft causes the shaft to be displaced such that the oil film on the side of the bearing opposite the applied force can become very thin. At high power transmission levels, the directional load can become so great that, in the limit, the floating bearing stops rotating within the housing. As a result, the load carrying capacity of the bearing drops and failure of the shaft bearing system can occur. Consequently conventional power turbines have some form of fixed bearing arrangement at the drive connection and of the shaft, typically a ball bearing assembly.
It is an object of the present invention to obviate or mitigate the problems outlined above.